SP-33: change NVDA punctuation/symbol level to a drop-down with strings as labels

Note: This commit is incomplete without the next one, which adapts the HTML code.
SP-33: Change NVDA punctuation/symbol level to a drop-down with strings as labels (part 2 of 2)

This should complete the change from slider to drop-down list for NVDA's NVDA punctuation/symbol level.
Merge remote-tracking branch 'cstrobbe/patch-12'

* cstrobbe/patch-12:
  SP-33: Change NVDA punctuation/symbol level to a drop-down with strings as labels (part 2 of 2)
  SP-33: change NVDA punctuation/symbol level to a drop-down with strings as labels